One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between down-hole locations themselves. For example, communication is desired by the oil industry to retrieve, at the surface, data generated down-hole during operations such as perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to down-hole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable down-hole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded digital signals.
One approach which has been widely considered for borehole communication is to use a direct wire connection between the surface and the down-hole location(s). Communication then can be made via electrical signal through the wire. While much effort has been spent on “wireline” communication, its inherent high telemetry rate is not always needed and very often does not justify its high cost.
Another borehole communication technique that has been explored is the transmission of acoustic waves. Whereas in some cases the pipes and tubulars within the well can be used to transmit acoustic waves, commercially available systems utilize the various liquids within a borehole as the transmission medium.
Among those techniques that use liquids as medium are the well-established Measurement-While-Drilling or MWD techniques. A common element of the MWD and related methods is the use of a flowing medium, e.g., the drilling fluids pumped during the drilling operation. This requirement however prevents the use of MWD techniques in operations during which a flowing medium is not available.
In recognition of this limitation various systems of acoustic transmission in a liquid independent of movement have been put forward, for example in the U.S. Pat. No. 3,659,259; 3,964,556; 5,283,768 or 6,442,105. Most of these known approaches are either severally limited in scope and operability or require down-hole transmitters that consume a large amount of energy.
It is therefore an object of the present invention to provide an acoustic communication system that overcomes the limitations of existing devices to allow the communication of data between a down-hole location and a surface location.